From Arkham Horror Wiki
Atlach-Nacha is an Ancient One. He first appeared in the Kingsport Horror expansion.
Combat rating: -5
Defenses: Magical Resistance and Physical Resistance
Worshippers: Atlach-Nacha is worshipped by the terrible Leng Spiders. Leng Spiders are fast, and their toughness is increased by 2.
Power: Web Between Worlds - While Atlach-Nacha stirs in his slumber, all gate openings are gate bursts. In addition, all Mystic cards are discarded without their special effects taking place.
Attack: The Investigators must, as a group, choose one Ally in play or one investigator. If an Ally is chosen, it is returned to the box. If an investigator is chosen, that investigator is devoured.
Doom Track: 13
Atlach-Nacha first appears in The Seven Geases (1934), written by Clark Ashton Smith.